While I agree that the software option (B4) is the one which comes closest to the real thing, I have to say that it's a bit awkward for a live situation (if you want to modify the drawbars settings you have to use a mouse or another similar device and that takes time).
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Just to point out that the B4 does respond to MIDI control and that allows me to push REGISTRATION MEMORY buttons on my PSR-740 and change the B4 condition. No need to touch the mouse. Of course, you have to have previously determine the paramters you want for that discrete way of operating the B4.